Why is it that it's mostly adults that die from measles rather than young babies?

Is it worse as an adult?

(got my info from yougov.uk)

Also I read that the second MMR jab was only introduced later, so there's lots of adults that only had one injection. It's spreading because of people travelling in europe. Why are people so worried about children when majority of children are vaccinated when it's actually spreading throughout adults and adults are more at risk?

There's lots of outbreaks but not many are actually dying in the UK/usa far as I'm aware.

The flu worries me more but in the 'do u think vaccines should be mandatory' debates, all the pro vaxxers say they should be but yet they don't want the flu vaccine. The flu kills thousands each year I don't get it.

Sorry if I sound dumb. I'm not anti Vax I'm just trying to learn about them as everyone keeps arguing on this site. I don't have kids and I am vaccinated and get the flu jab